Ammonium nitrate (AN), with the chemical formula NH4NO3, is a worldwide available chemical compound, produced in millions of tons every year. Its main use is as a fertilizer, and therefore it is widely available and easily accessible in many countries around the world. Its second main use is for the manufacture of explosives, that is by definition very closely controlled.
An undesired use is for the production of home-made explosives (HME's) such as a mixture of ammonium nitrate and fuel oil, also called ‘ANFO’, or a mixture of ammonium nitrate and nitro methane, also called ‘ANNM’. The recipes of the HME's are publically available, for example on the Internet. In practice, to produce an HME on the basis of an AN-based fertilizer is rather straightforward, and misuse of AN-based fertilizer into explosives occurs too frequently, especially in the context of terroristic attacks, but also by farmers for various blasting purposes, by children or students experimenting with explosives, for example in the production of (illegal) fireworks.
To limit its misuse, many countries have banned the use of pure AN as a fertilizer by limiting the nitrogen content of AN-based fertilizer. A typical fertilizer formula is the so-called calcium ammonium nitrate (CAN) which is a mix of ammonium nitrate with a carbonaceous filler such as limestone or dolomite and with a maximum AN-content of 80 weight %. Different CAN grades exist, having a varying amount of nitrogen.
Moreover, such a CAN-fertilizer has the advantage that it is an excellent fertilizer, bringing nitrogen to the plant in a readily available form. It also brings some valuable calcium and/or magnesium. Furthermore, it is well-balanced regarding soil pH, avoiding the natural acidification of the soil caused by some nitrogenous fertilizers such as urea, this due to the conversion of ammonium nitrogen into nitrate nitrogen, to be assimilated by plants. Liming is in fact already included in the product.
Many other AN-based fertilizer exist, not only straight nitrogen (N) fertilizers (with different degrees of N-dilution by a filler or containing secondary nutrients such as e.g. sulphur), but also NPK (indifferently NPK, NP, NK) and especially high N-NPK fertilizers. All of them can be misused, but of course, the preferred choice for a potential misuse would logically be to opt for a fertilizer which is the most concentrated in ammonium nitrate, i.e. a straight AN-fertilizer or a CAN-fertilizer. However, the current invention is applicable to all AN-based fertilizers, as long as they contain a certain amount of ammonium nitrate.
There are basically two routes to convert an AN-based fertilizer such as a CAN-fertilizer into AN, suitable for home-made explosives, i.e.:    (1) A dry route (also called grinding route), wherein the granules or brills of the AN-based fertilizer such as a CAN-fertilizer—are simply crushed and later mixed with other compounds such as oil or aluminium powder;    (2) A wet route (also called solution processing route), wherein the optionally grinded granules or prills of the fertilizer are first dissolved into water, the solution is allowed to settle and the insoluble components such as limestone or dolomite can be filtered out (AN is highly soluble in water (1920 g/l at 20° C.); calcium and magnesium carbonates are poorly soluble in water). In that way, one can recover an aqueous solution of ammonium nitrate, that can be evaporated to get a concentrated ammonium nitrate product. This concentrated ammonium nitrate product is more concentrated than the ammonium nitrate product obtained by means of the dry route. The wet route requires more process steps, but definitively allow obtaining a more powerful home-made explosive.
Therefore, there exists a need to limit as much as possible the potential misuse of AN-based fertilizers, in particular of CAN-fertilizers, as a precursor for an explosive on the one hand, without hampering their legitimate use by farmers for food production on the other hand, especially when the wet route is used to convert an AN-based fertilizer.